Abstract
The variation of crack shape factor (a/c) during propagation has an important role on calculation of the stress intensity factor (SIF), where imprecise estimation of this latter can lead us to an inaccurate prediction of failure probabilities. For the case of external cracked surface pipeline carried oil and gas products, such estimation can results undesired fault decisions like excessive repairs action or inspection planning. In this paper, the structural integrity analysis of pipeline with semi elliptical crack on the external surface is evaluated. Reliability calculations expressed in term of reliability index β are carried out based on Monte Carlo simulation and First Order reliability Method (FORM). The crack shape factors are varied to cover a variety of geometries from shallow to deep cracks while fatigue crack growth is assumed in depth direction. For SIF estimation, surface and deep point are considered. Results of the analysis indicate that for a constant crack depth a/t, the reliability index is strongly affected by a/c ratio and the trend is not similar in deep and surface point for different crack shapes.
Highlights
O ne of the strategic energy resources are oil and natural gaz. they have the advantage to ensure a continues and reliable energy supply with an acceptable cost
According to the variation of reliability index calculated in deep point, this later is greatly influenced by the crack shape and depth ratio
Shallow cracks have lower reliability index followed by deep cracks .As consequences, it seems that shallow cracks are more susceptible to cause fracture deep cracks
Summary
O ne of the strategic energy resources are oil and natural gaz. they have the advantage to ensure a continues and reliable energy supply with an acceptable cost. The comparison shows that analytical method based on local SIF (Kloc) calculated according to Newman and Raju formula [3] and the assumption of Cc=0.9Ca (Cc and Ca are fatigue crack growth rate in surface and deep point) is the best solution over the full range of initial crack shape values compared to experimental data with standard deviation less than 0,07. Boukharouba [4] reviewed proposed prediction model of crack shape factor evolution with depth ratio in fatigue cracking test for the following cases: tension plate, three point bending plates and tube under internal pressure. They conclude that a/c ratio passes by maximum (growth in deep direction is more rapid comparing to surface direction) remains steady following horizontal asymptote during crack growth Another important conclusion is that behavior of a/c evolution is similar in thick plate under tension and thick tube under internal pressure. A case study of pipeline with axially external defect is applied
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